1. Field of the Invention
The present invention relates to signal transmission cables that preferably define and function as a band-elimination filter and/or a signal transmission line connecting two circuits.
2. Description of the Related Art
In the related art, some small electronic devices performing wireless communications such as cellular phones and the like include a plurality of mount-type components such as boards and the like in their casings. In such a case, the plurality of boards is sometimes connected by flexible flat cables.
For example, a small electronic device performing wireless communications includes an antenna board and a front end board (refer to PCT International Publication No. WO 2005/114778 A1). On the antenna board, an antenna is formed for transmitting and receiving high-frequency signals. On the front end board, a front end circuit is formed. The front end circuit generates high-frequency signals to be radiated from the antenna, and amplifies and demodulates high-frequency signals received with the antenna. The antenna board and the front end board are generally placed at locations separated from each other in the casing. Thus, a flat cable is used to connect the antenna board and the front end board.
Further, in current small electronic devices, the antenna is configured to be able to transmit and receive high-frequency signals in a plurality of communication bands, and shared by front end circuits of respective communication bands. Thus, a high-frequency filter is typically connected between the antenna and each front end circuit to establish isolation between the front end circuits of the respective communication bands.
Further, in small electronic devices of the related art, such a high-frequency filter is mounted or formed on the front end board on which the front end circuit is formed. In a case where the high-frequency filter is mounted on the front end board, an inductor and a capacitor constituting the high-frequency filter are mount-type elements, as is the case with an inductor element described in Japanese Unexamined Patent Application Publication No. 2000-196391, for example. In a case where a high-frequency filter is formed on the front end board, an inductor and a capacitor constituting the high-frequency filter are realized with inner layer electrode patterns in the front end board.
However, in the foregoing related art configuration, the high-frequency filter is formed on the front end board. As a result, a shape of the front end board becomes larger by the volume of the high-frequency filter thus formed. This put a limitation on downsizing of the front end board.
Further, when an attempt is made to downsize the high-frequency filter mounted or formed on the front end board, the line width of the inductor is reduced, or constraints are imposed on the shape of the inductor. This degrades the Q value of the inductor, and accordingly degrades the Q value of the high-frequency filter. Accordingly, in some cases, desired filter characteristics (bandpass characteristic or attenuation characteristic) may not be achieved. For example, in a case where a band-elimination filter is formed, a steep attenuation characteristic cannot be achieved.